The variety of audio/video sources has increased in number in recent years. In general, a selector function is added to an AV (Audio Visual) amplifier or the like which is used as a component in building an AV system for handling these sources in an integrated control manner.
The selector receives audio signals generated typically by audio sources such as a compact disc player referred to hereafter simply as a CD player, a minidisk player referred to hereafter simply as an MD player and an AM/FM tuner in addition to video and audio signals generated by video/audio sources such as a television tuner referred to hereafter simply as a TV tuner, a video recorder referred to hereafter simply as a VTR, a laser disk player and a video CD player. By carrying out predetermined operations, a signal selected from the audio and video/audio signals supplied by these sources to the selector can be output.
Let, for example, video and audio signals supplied from a TV tuner be selected at the present time. When it is desired to switch from these video and audio signals to other signals, a select key or the like provided on an operation unit is operated. In this way, it is possible to switch from these video and audio signals to other signals at the same time by carrying out one operation.
In addition, video and audio signals can be selected from video and audio sources independently of each other. To put it in more detail, for example, it is possible to select an audio signal supplied from a CD player and a video signal generated by a TV tuner as a BGV (Back Ground Video) at the same time.
In this case, however, if video and audio signals are selected in a completely independent manner, one signal must be selected after another, giving rise to poor operability. This is because the signals are selected one after another by carrying out two seperate select operations. In order to improve the operability, an audio select mode is provided. In the audio select mode, with a TV tuner producing video and audio signals, an audio signal selected from, for example, a CD player or an MD player outputting only an audio signal can be selected so as to switch the AV system from the audio signal supplied by the TV tuner to the audio signal generated by the CD player or the MD player, leaving the video signal generated by the TV tuner untouched as it is.
By the way, a broadcasting system which allows character data of an independent program to be transmitted by superposition typically on a video signal of a TV broadcast has been implemented in recent years.
In a teletext, one program comprises typically several pages of character data. A teletext receiving unit receives such a superimposed video signal of the TV broadcast and demodulates the signal to produce the character data. Subsequently, a video signal conveying character transmission screens corresponding to the pages is generated. Then, the video signal is supplied to a monitor unit for displaying the character transmission screens on a monitor screen, allowing various kinds of information to be obtained.
In some cases, the teletext may include audio information added as BGM (Back Ground Music) of the screen as is the case with "karaoke ". In such a case, the teletext receiving unit outputs video and audio signals. Thus, by supplying the video and audio signals supplied by the teletext receiving unit to a selector, it is possible to watch screens of the teletext and listen to the audio output thereof in the AV system.
For example, let the user temporarily select a teletext by means of a selector in order to watch a weather forecast or traffic information while the user is listening to music produced by a CD or MD player. Since a teletext comprises signals generated by both audio and video sources, however, switching both video and audio signals being reproduced by an AV system to the signals of the teletext will give rise to a problem that the audio signal being played back by a CD or MD player is inadvertently suspended and the audio signal of the teletext is reproduced instead, resulting in an unpleasant feeling.
In this case, the AV system can of course be switched back from the audio signal of the teletext to the audio signal reproduced by the CD or MD player. In this case, in order to watch the weather forecast or the traffic information of the teletext, two seperate switching operations must therefore be carried out as described above, giving rise to a problem caused by the troublesome operation to switch back to the audio signal reproduced by the CD or MD player.